Switching-mode power supply (SMPS) is widely used to convert one voltage into another voltage which supplies a load. A SMPS contains a main switch and by controlling the switching action of the main switch, the output voltage can be regulated at a desired level.
A Buck converter is one type of SMPS which converts a Direct Current (DC) input voltage into a lower DC output voltage. FIG. 1 shows a schematic diagram of a buck converter 100 as a prior art. The buck converter 100 comprises a controller 11, a main switch M, an inductor L and an output capacitor C. The main switch M of buck converter 100 is regulated by a pulse width modulation (PWM) driving signal generated by the controller 11, and accordingly, the output voltage Vout of the buck converter is regulated. When load 12 varies, the output voltage Vout is disturbed and by sensing the output voltage Vout and offering an output feedback signal FB to controller 11, the duty cycle of the PWM signal is adjusted and Vout is regulated to the desired level.
Ripple mode control is one type of control method for SMPS which controls the main switch based on a ripple component of the output voltage. For buck converter 100, since the main switch M is controlled by the PWM signal, the buck converter will generate a ripple current IL flowing through output inductor L. This ripple current will further generate a ripple component of Vout due principally to the equivalent series resistance (ESR) of output capacitor C. The ripple component of Vout equals IL*RESR. When Vout with ripple component is lower than a reference voltage, main switch M is turned on. To achieve stable control, a large ripple component is needed. However, it is not expected because an output voltage with ripple component will affect the working of load 12. And further, a large ripple component requires a large ESR and the power efficiency decreases. Thus, a small ESR is desired and additional circuit is required to generate a ramp voltage signal similar to inductor current IL.
A compensation circuit 21 is disclosed for ripple mode control in FIG. 2. The compensation circuit 21 comprises a resistor R1 and a capacitor C1 coupled to two ends of output inductor L. Compensation circuit 21 generates a ramp voltage signal Vramp similar to the shape of the output inductor current IL. However, when this compensation circuit 21 is an external circuit adopting discrete components, the components will consume a large space and the cost will increase. In some applications, this compensation circuit 21 is integrated with the control signal generator 22 in a die. However, when the compensation circuit 21 is built in the semiconductor die, the values of R and C are fixed and the slope of the generated ramp voltage signal can not be adjusted and thus can not be adapted to the various applications.
Accordingly, an improved method of generating ramp signal for ripple mode control is desired to at least overcome part of the above mentioned deficiencies.